The present invention relates to an information reading device such as a bar code reader. More particularly, the present invention relates to a laser bar code scanner using a hologram disk. The present invention also relates to a reconstructed hologram which is used as a hologram scanner or an ornamental hologram. The present invention further relates to a grating element used in a scanner such as a bar code reader and a laser printer and a method for producing the same.
Japanese Patent Application Laying Open (KOKAI) No. 53-117333 discloses an example of an information reading device used as a bar code reader or the like which reading device comprises a convergent type hologram disk. This prior art information reading device reads information of bar code in such a way that first a laser beam from a laser source passes through a convergent hologram disk, then the converged beam is irradiated to a scanning surface of a bar code to be read, and after that reflection rays reflected from the bar code are guided back to the convergent hologram to pass therethrough again so that the converged reflection rays are guided to and detedted by a photo-sensor.
There is also another known information reading device comprising a feed back optical system in which a scanning hologram which is the same as a convergent hologram is used, instead of comprising the above-mentioned reflection type optical system of the convergent hologram disk.
In the above-mentioned prior art information reading devices comprising the hologram disk, it is possible to compound various functions such as a converging function for converging the irradiating beam directed to the bar code to be scanned, a deflection function for deflecting the scanning beam by optical elements such as mirrors disposed on the optical path of the beam, an optical receiving function for receiving the reflection light reflected from the bar code, and a raster scanning function. This makes it possible to reduce the number of parts of the scanner and increase the scanning reliability thereof. However, the prior art information reading devices do not sufficiently satisfy the need for compact readers, especially need for thin readers.
Also, in accordance with the prior art information reading device, the optical path of the reflection beam is separated from that of the irradiating scanning beam by such a way that a mirror having an aperture at the center thereof is disposed on the optical path of the scanning beam inclining with respect to the scanning beam, so that the scanning irradiating beam passes through the aperture while the reflection rays are reflected by the mirror. The mirror reflects the rays reflected from the bar code toward an optical sensing means to detect the optical intensity of the reflection beam. However, since the aperture is arranged at the center of the mirror, a large part of the most intensive rays positioned at the center of the reflection beam is eclipsed from the mirror. As the result, the optical amount of the reflection beam to be detected is decreased, which impairs the optical detection efficiency of the reflection beam and lowers the reliability of the detection.
Also, as mentioned before, the information reading devices in accordance with the prior art are not very satisfactory in terms of realizing a thin scanner.
In order to meet the need for the thin and compact scanner, an information reading device is proposed which device comprises a semiconductor laser beam source for the source of scanning beam which source is constituted as one unit including the hologram disk and other optical systems so as to simplify the structure thereof. However, this proposed information reading device utilizes the semiconductor laser of invisible infrared having a wavelength more than 780 nm. Therefore, the position of the scanning beam spot can not be confirmed by the human eyes, which is inconvenient to adjust the scanning point of the laser beam at the time of operating the reading device.
To cope with this problem, it is proposed to prepare an LED laser of visible light and irradiate the visible laser beam to the bar code surface to be scanned using a galvano mirror, whereby enabling to adjust the scanning point of the laser beam by confirming the position thereof by the operator's eyes. However, this proposed reading device requires the specified laser beam source separated from the other optical systems, which increases the number of optical parts and makes the structure complicated as well as to rise the cost thereof.
In accordance with the prior art information reading device, the bar code is read by the following way with the use of the laser beam scanner comprising a hologram disk.
The laser beam is irradiated to the rotating hologram disk having a plurality of sectors so that the laser beam is deflected and swung by each sector whereby scanning the bar code surface to be read. The scanning beam is reflected by the bar code surface and deflected by the same hologram disk so that the hologram is reconstructed and detected by an optical sensor whereby the bar code is read.
The above-mentioned conventional hologram disk comprises a hologram having linear gratings. Therefore, when the angle of incidence does not meet the Bragg angle condition, a large astigmatism arises. Also, the beam spot shape is distorted on the position out of the scanning line center even if the Bragg angle condition is satisfied.
On the other hand, when the hologram disk is made from a hologram having circular arc gratings, if the wavelength of construction beam for making the hologram is different from that of reconstruction beam for reading the hologram, the astigmatism also arises and the beam spot shape is distorted on the portion out of the scanning line center.
Therefore, when such holograms are applied to the laser bar code scanner, the reading area which can be covered by the scanner is very narrow since the width of bar code is very small (about 0.2 mm).
In order to widen the reading area of the scanner, it is proposed to produce the hologram with the use of construction beams arranged by a complicated optical system so as to remove the astigmatism at the time of reconstruction of the hologram. However, the hologram made by this method requies a large rotation angle to reconstruct the hologram so that the scanning beam spot is distorted at a position where the scanning beam is largely swung.
Also, to avoid the astigmatism as possible, it is proposed, in Japanese Patent Application Laying Open (KOKAI) No. 60-238885, to produce a hologram with the use of a spherical wave having a wavelingth of .lambda. and a plane wave having the same wavelength of .lambda. as construction waves which are arranged by passing through a prism having a predetermined apex angle and refractive index at a specified angle of incidence. However, the arrangement of such a prism and adjustment of the angle of incidence of the construction beam are not easy with regard to this method. Also, the astigmatism still remains in a portion of large scanning angle.